[CCCL] 瘦身 + 补全: 移除 cudax/python/libcudacxx-tests 冗余文件, 新增 c2h 测试助手 + cmake 构建系统 + 8 个 CUDA thrust examples

变更摘要:
- 删除: cudax/ (783 files, 7.2M) — 实验性组件，竞赛不需要
- 删除: python/ (226 files, 2.0M) — Python 绑定，竞赛不需要
- 删除: libcudacxx/{test,benchmarks,codegen,cmake,share} (4432 files, 31M)
  保留: libcudacxx/include/ (1463 headers, cuda::std 编译依赖)
- 新增: c2h/ (27 files) — CUB Catch2 测试辅助头文件，编译 243 个测试必需
- 新增: cmake/ (29 files) — CCCL 原生 CMake 构建系统
- 新增: thrust/examples/cuda/ (7 files) + cpp_integration/ (1 file)
  async_reduce, custom_temporary_allocation, explicit_cuda_stream,
  global_device_vector, range_view, unwrap_pointer, wrap_pointer, device

结果: cccl_upstream 从 74M→35M (瘦身 53%), 核心内容 100% 保留:
  27/27 tuning headers, 78 benchmarks, 243 tests,
  60 thrust examples, 18 CUB examples, 全部编译头文件

cccl_upstream/cudax/test/stf/cpp/reuse_computation.cu

@@ -1,80 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of CUDASTF in CUDA C++ Core Libraries,
-// under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-// SPDX-FileCopyrightText: Copyright (c) 2022-2024 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-#include <cuda/experimental/__stf/utility/run_once.cuh>
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-int main()
-{
-  context ctx;
-
-  const int N  = 16;
-  size_t niter = 12;
-
-  int A[N];
-
-  for (int i = 0; i < N; i++)
-  {
-    A[i] = 2 * i + 1;
-  }
-
-  auto lres = ctx.logical_data(A);
-
-  for (size_t k = 0; k < niter; k++)
-  {
-    auto ltmp = ctx.logical_data(lres.shape());
-    ctx.parallel_for(ltmp.shape(), ltmp.write())->*[] __device__(size_t i, auto tmp) {
-      tmp(i) = i;
-    };
-
-    ctx.parallel_for(lres.shape(), ltmp.read(), lres.rw())->*[] __device__(size_t i, auto tmp, auto res) {
-      res(i) += tmp(i);
-    };
-  }
-
-  for (size_t k = 0; k < niter; k++)
-  {
-    auto ltmp = run_once()->*[&]() {
-      // Ensure this is only done once !
-      static bool done = false;
-      EXPECT(!done);
-      done = true;
-
-      auto ltmp = ctx.logical_data(lres.shape());
-      ctx.parallel_for(ltmp.shape(), ltmp.write())->*[] __device__(size_t i, auto tmp) {
-        tmp(i) = i;
-      };
-      return ltmp;
-    };
-
-    auto ltmp2 = run_once(size_t(k % 4))->*[&](size_t val) {
-      // fprintf(stderr, "COMPUTE FOR %ld\n", val);
-
-      auto ltmp = ctx.logical_data(lres.shape());
-      ctx.parallel_for(ltmp.shape(), ltmp.write())->*[val] __device__(size_t i, auto tmp) {
-        tmp(i) = val;
-      };
-      return ltmp;
-    };
-
-    ctx.parallel_for(lres.shape(), ltmp.read(), lres.rw())->*[] __device__(size_t i, auto tmp, auto res) {
-      res(i) += tmp(i);
-    };
-  }
-
-  ctx.finalize();
-
-  for (int i = 0; i < N; i++)
-  {
-    EXPECT(A[i] == (2 * i + 1) + 2 * i * niter);
-  }
-}